Electromagnetic emissions by electronic equipments and appliances are highly undesired, because they are a cause of disturb (interference) of the operation of other electronic equipments. In some cases, electromagnetic emissions are even harmful to the human body.
Many national and international regulatory authorities have therefore issued prescriptions (generally referred to as ElectroMagnetic Interference or, shortly, EMI regulations) aiming at setting maximum limits to the level of electromagnetic emissions by electronic circuits and apparatuses. Reducing as far as possible the level of electromagnetic emissions is therefore a major issue that electronic circuit designers have to cope with.
In particular, a source of electromagnetic emissions in electronic circuits are signals subjected to relatively fast transients. For example, in digital electronic circuits, clock signals, data buses, address buses and control signals are typically subjected to sharp-edge transitions. Electromagnetic radiation is primarily associated with the fundamental, the third and the fifth harmonics of these signals.
The problem of electromagnetic emissions is exacerbated by the constant increase in performance of electronic apparatuses, translating into increasingly higher clock speeds in the electronic circuits: by way of example, in the past few years microprocessor clock rates have experienced an impressive ramp-up.
Classical techniques for reducing electromagnetic emissions by electronic apparatuses, including for example coaxial wires, shielded cables, shielded casings, shielding paints, are directed to contain the generated electromagnetic radiation, more than to limit the level thereof. The main drawbacks of these techniques are bulkiness, relatively high cost, and scarce flexibility.
A more recent and quite effective technique for reducing the level of electromagnetic emissions by acting directly on the source of the electromagnetic radiation is spread-spectrum modulation of fast-varying signals, or, shortly, Spread-Spectrum Technique (SST). According to the SST, the fast-varying signal is modulated in frequency, in a carefully controlled way, so that the spectral energy of the signal is spread over a wider frequency range, and thus the spectral energy peaks are attenuated.
It has been experimentally proved that using the SST, the level of electromagnetic emissions can be lowered to an extent ranging from 5 dB to 20 dB, depending on the degree of modulation of the fast-varying signals.
The SST is mainly applied to square-wave signals, such as clock signals encountered in several digital electronic circuits. Spreading the spectral energy of the fundamental harmonic over a tightly controlled frequency range also causes the spectral energy of the higher harmonics to be distributed over a wider frequency range.
In the practical implementation of the SST, care needs to be adopted to ensure that the inevitable change in the clock signal frequency is totally transparent to the electronic circuit or system; in particular, it is important to ensure that both cycle-to-cycle jitter and peak-to-peak jitter remain within the electronic circuit or system specifications.
Phase-Locked Loop (PLL) circuits are typically used for generating clock signals of desired frequencies starting from a reference clock signal, produced for example by a crystal oscillator. Several solutions have been proposed for implementing the SST in a PLL clock signal generator. Generally stated, all these solutions involve acting in some way on the PLL circuit in order to modulate in frequency the generated clock signal.
For example, in the U.S. Pat. No. 5,631,920 different embodiments of a spread-spectrum clock generator are described. In one embodiment, two Voltage-Controlled Oscillators (VCOs) are used, one inserted in a conventional PLL and another driven by a signal that is the sum of the PLL output signal and of a modulation signal; in another embodiment, the reference frequency inputted to the PLL is varied; in still another embodiment, a variable-divide factor frequency divider is used in the feedback path of the PLL.
In the U.S. Pat. No. 6,294,936, modulation is achieved by injecting a modulation signal into the PLL's VCO.
The Applicant has observed that a problem with this technique is that it does not enable to carefully control the modulation amount; the VCO gain is in fact hardly predictable, especially in integrated circuits, because it depends on several parameters such as the temperature.
In the U.S. Pat. No. 5,943,382, a dual-loop spread-spectrum clock generator is described, with a master PLL and a slave voltage-locked loop producing a modulated voltage which is added to a voltage produced by the phase comparator of the master PLL for driving the VCO.
Also in this case, the modulation index is poorly controlled.
The Applicant has observed that in the prior-art spread-spectrum clock generators that relies on a PLL, with a modulation signal directly driving the VCO, the modulation index cannot be tightly controlled, because the loop, due to the relatively narrow bandwidth, does not work in locked condition; the continuous variation of the clock frequency causes the PLL to be always in a capture transient.